Programming the Zynq 7000 with Vivado 2019.2 and Vitis

I purchased a Zynq 7000 development board recently and wanted to play with it for quite some time now. Finally, the lock-down due to the corona virus pandemic gave me some time to put my hands on Zynq SoC development. In the past, I had spent most of my time developing RTL code for various projects. Ever since Zynq SoC had come out on the market, it had interested me mainly because of the tight integration of the programmable logic and the dual core ARM applications processor. The combination of the two has lead to integration of so many system components in various projects out there.

The best example I know in front of me is the GW Instek GDS-1102, GDS-1104 series of oscilloscopes. The entire oscilloscope has its brain embedded into a single Zynq SoC. The processing system handles the display related and user facing side of the things. On the other hand, the programmable logic interfaces with the high speed ADC and does all sort of digital processing you would expect an oscilloscope to do. If you wish to know more, head out to the EEVBlog video where Dave tears down one of the GW Instek oscilloscopes and explains everything very clearly.

Do not expect a very complicated design in this post. I have saved something good for a later date.

Block design

Consider this as a project 0 for someone who wishes to start with Vitis. A lot of projects out there related to Zynq use quite older version of Vivado and SDK. Although, most of the things remain the same, there may be a few changes here and there.

In this very simple project, I will interface 4 LEDs and 4 switches to the Zynq Processing system. The interface would be standard AXI4 because that is the only way you can interface programmable logic to the ARM processor.

It would take me ages to make a tutorial explaining every small step in the GUI. Instead, I am providing you with the tcl script that you can run and make life easy.

First, create project, select the right board and then create a new block design. Make sure you run this all in the Vivado tcl console. Also make sure you change the project according to your need.

Now, add the Zynq Processing system IP block and the AXI GPIO block. Then run the automation to connect it all together.

Now connect the DDR and the IO ports.

Once you have run this, you should end up with a block diagram that looks something like the following.

The configurations

Now, we need to configure various things in the block design. If you are using one of the standard development boards, things would be quite simple. In case you are using a custom board, you will need to manually set a lot of things. In my case, I am using a Zybo Z7010 board which has 4 gpio pins set as output and 4 as input. The moment I added the AXI GPIO block, it automatically assigned it to those pins.

If you are using a custom/non-standard board, the board interface setting should be set to “Custom”. Next, switch to the IP Configuration tab and set everything manually.

If you wish to set the system clock, you can double click on the Processing system block and do so.

We cannot forget mapping the GPIO signals to the hardware pins. That is done through the constraints file. If you are using a standard board, you can simply place the given constraints file and un-comment the necessary pins.

Observe carefully, the part where it says “[get_ports { led[0] }]”. The signal “led” is the name of our GPIO port in the block design. If you happen to make a mismatch in this naming, your signals will never see the real world. Make the settings in the port properties area as shown below.

Synthesis

Looking at the screenshots you may be wondering, where did the blinky_1_wrapper come from? Until now, you should only see the block design and nothing more. Now, right click on the block design and press “Generate output products”. Once that is done, right click again and press “Create HDL wrapper”. Finally, your project space should look similar to mine in the screenshots.

Go the bottom and press “Generate bit stream”. This will initiate the synthesis and implementation process. When the bit stream generation, go to “File > Export > Export hardware” and make sure to “Include bit stream“. Our job with Vivado is done here. We now move to the Xilinx Vitis IDE to configure the rest.

Vitis IDE to program the Zynq processing system

Start your Vitis IDE and import the .xsa file that got generated when we exported the hardware from Vivado. To do this, do the follwing: “File > New application project”. Make sure your project name has no spaces.

Click on “Create a new platform from hardware (XSA)” and then press the “+” icon to import the hardware file we generated in Vivado.

Proceeding further, select “Hello world” or an empty application project. Doing so, we will have two projects created in Vitis IDE. One project is the board support package that contains all the required drivers and definitions. The other project is our application project that will be building on top of the provided BSP.

Expand the blink1_system project and open the “helloworld.c” file. Our aim here is to simply make use of the GPIO peripheral we just created in Vivado and toggle some LEDs.

Some C programming

There are two places where you can see the base address of our GPIO block. One is by looking at the properties of the GPIO block in the Vivado design and the other place is looking at the platform.spr file in Vitis IDE.

In order to send data over AXI, Xilinx provides us with Xil_in and Xil_out functions. We can either use those or go for the XGpio functions defined in the xgpio.h file.

Two main functions will help us write to the GPIO peripheral. Firstly, let us initialize a GPIO instance by using the function XGpio_Initialize which takes two arguements. One of the arguments being the instance pointer and the other is device ID. You will find the device ID in xparameters.h file.

We then set the direction of the GPIO using XGpio_SetDataDirection.

Remeber, that we have 4 output pins to drive. All of them are connected to a single GPIO channel called “GPIO” and “GPIO2”. If you don’t remember, go back to Vivado and check it out.

The address to access each channel is fixed. So how do we drive individual LED pins? By setting individual bits in the output register.

Now, let us drive these leds and see if that works.

We are almost done. Click on the debug icon and create a debug configuration. Finally, build your code and debug/Run. You should see dancing LEDs on your board.